Conventionally, pistons for internal combustion engines have been produced through permanent mold casting. Firstly, molten aluminum alloy is teemed into a casting mold to thereby mold the alloy into a piston preform. Subsequently, the resultant preform is subjected to heat treatment as required, such as intentional aging, and then subjected to mechanical working as required to thereby produce a final product.
Recently, in some cases, pistons for internal combustion engines have been produced through forging. Molten aluminum-silicon alloy is subjected to continuous casting to thereby form a billet for extrusion; the billet is subjected to heat treatment (homogenizing treatment) in order to attain uniform distribution of internal stress generated by segregation of solute elements or shrinkage during solidification; and the resultant billet is formed into a round bar of small diameter through extrusion. Alternatively, molten aluminum-silicon alloy is subjected to continuous casting to thereby form a continuously cast bar of small diameter; the resultant cast bar is subjected to homogenizing treatment; and the resultant bar is subjected to machining to thereby form a round bar of small diameter. The thus formed round bar of small diameter is cut into pieces serving as a forging material. The forging material is preliminarily heated, and then forged into a piston preform by use of a hot-forging machine. Subsequently, the preform is subjected to heat treatment, such as intentional aging, and then subjected to mechanical working to thereby produce a final product (i.e., a piston). In accordance with use of the piston, in order to improve wear resistance and heat resistance, the head of the piston or a portion of the side wall of the piston between a top ring and the head may be subjected to alumite treatment or coating formation treatment.
Recently, demand has arisen for further improvement in fuel economy of internal combustion engines employed in, for example, automobiles. In order to meet such demand, attempts have been made to reduce the weight of an automobile body, and lightweight engines have been developed. For example, pistons employed in engines have been produced from aluminum, and pistons of thin wall structure have been developed.
Meanwhile, there has arisen demand for pistons of high quality that meet requirements of high-performance engines.
When a piston is produced through a conventional permanent mold casting method, because of technical limitation imposed on the casting method, the thickness of a skirt section is difficult to reduce. Therefore, in general, the cast piston is subjected to machining to thereby reduce the thickness of the skirt section. When a piston is produced through casting, the metallographic structure of the piston become coarse as a result of low solidification rate during casting, so that the resultant piston exhibits good mechanical workability. However, since variation in thickness and dimension between pistons formed through casting is large, dimensional accuracy of final products is difficult to control. Furthermore, internal defects, such as cavities and microshrinkage, may arise in a piston produced through casting, thereby lowering its strength. Therefore, in order to improve the strength of the piston, the entire wall of the piston is thickened and the thickness of a rib is increased, thereby making a piston produced through casting unsuitable for use in an engine of high performance. In addition, variation in performance between pistons becomes large due to thickening of the wall of the pistons. In view of the foregoing, producing engines of reliable performance requires further improvement of pistons.
Meanwhile, when a piston is produced from a forging material through forging, the thicknesses of sections constituting the piston become uniform, since the forging material contains substantially no internal defects, and the forging material has reliable mechanical characteristics. Therefore, a piston of reliable quality can be produced through forging. However, since the forging material has a fine metallographic structure, mechanical workability of the material is not satisfactory, although the material is suitable for forging of a thin, long section, such as a skirt section. For example, since chips of continuous form, as contrasted to fragmental form, are generated during mechanical working, manageability of the chips is impaired, resulting in poor productivity. In addition, the surface roughness of an oil ring groove section of a final piston product that has undergone mechanical working is not satisfactory. When continuous casting is employed, in order to prevent occurrence of cracking attributed to solidification-shrinkage stress generated in a cast ingot during casting, a limitation is imposed on the composition of the alloy to be produced. Therefore, an alloy of desired composition which serves as a forging material capable of providing a piston exhibiting higher strength, higher wear resistance and higher strength at high temperature than required cannot be produced easily.
In view of the foregoing, the present invention has been developed, and the object thereof is to provide a forged piston for an internal combustion engine, including an oil ring groove exhibiting improved mechanical workability (e.g., the oil ring groove exhibiting reliable dimensional accuracy in terms of surface roughness and flatness), a head section having excellent mechanical characteristics (e.g., a head surface and a piston pin portion constituting the head section exhibiting excellent mechanical strength characteristics at high temperature), a skirt section exhibiting excellent forgeability, and an oil ring groove section exhibiting reliable wear resistance.